Yarn-feeders are known, which comprise a stationary drum on which a motorized flywheel winds a plurality of yarn loops forming a weft stock. Upon request from a downstream machine, typically a circular/rectilinear knitting machine of a conventional type, the loops are unwound from the drum, then pass through a weft-braking device which controls the tension of the yarn, and finally are fed to the machine.
The yarn feeders of the above type are well-known to the person skilled in the art and have the main aim of maintaining the amount of yarn stored on the drum substantially constant apart from the yarn-drawing speed of the downstream machine, while minimizing the tension of the delivered yarn. To this purpose, the yarn feeder is provided with various sensors connected to a control unit. One of these sensors, in particular, generates at least one pulse per each unwound loop and may be, e.g., an optical sensor, a piezoelectric sensor, and the like. This sensor cooperates with the other sensors to optimize the yarn-winding speed of the flywheel in order to maintain the amount of yarn stored on the drum constant.
With the conventional systems, another sensor is arranged between the feeder and the knitting machine for detecting the stop of the yarn, which circumstance may occur in case of breaking of the yarn or unhooking of the yarn from the needles of the machine. In this case, the control unit commands the stop of the machine in order to prevent defects in the finished article and to avoid the weft tube of the article under processing to detach, which circumstance, as known, requires the laborious, time-consuming operation of re-inserting the yarns forming the article into the machine.
As known, the above yarn-breaking sensors may be either mechanical or electronic.
The advantage of the mechanical sensors is that they are less expensive, but they are less effective in terms of quickness of response; moreover, in operation, they graze the yarn by a sensing arm, thereby interfering with the yarn-feeding tension and consequently affecting the accuracy of the tension-controlling system.
The advantage of the electronic sensors is that they are more effective in terms of quickness of response and, in operation, they do not interfere with the tension of the unwinding yarn because the motion of the yarn is detected by a photoelectric sensor. However, these electronic sensors are very expensive and they need installing and wiring an additional supplying/communication circuit, with consequent rise in costs and in the complexity of the detecting system.